I was not familiar with the KERS acronym either but note that the Tesla system recovers potential (gravitational) energy too. It is this that has given me W/mi of 160 on one trip and even -146 on one notable occasion.
It is critical that people understand the the "motors" used in the Tesla cars are symmetrical 4 quadrant machines which means that they can produce or absorb torque when running in either direction. What they do is determined by the current that flows in the stator windings and that is controlled by a set of 6 switches which are turned on and off in the proper order, at the proper speed and at the proper duty cycle to effect the desired torque result. I should also state clearly here that I have never seen any Tesla documentation supporting this description. It assumes that Tesla is using modern 3 phase motor control techniques and the evidence certainly seems to support this. Assuming this description to be reasonably accurate then the design would capture nearly all the energy that is available to be captured. That which has been lost to heating of the air, the tyres, the road bed, the inverter/rectifier and potential energy is not recoverable (except for the potential when you go back down the hill). The generator is not 100% efficient so there will be some losses there but Tesla has done an excellent job of tweaking. The SWPMMs, for example, eliminate rotor I^2R and hysteresis losses. The inverter/rectifier (the 6 switches) isn't 100% efficient either. Some heat is dissipated during the transition from on to off. There may be some room for improvement there (by developing transistors that can switch faster for example) but they are working well above 90% and there just isn't that much more efficiency to be garnered. But every little bit does count and I am sure Tesla and the other manufacturers are looking at ways to get even a fraction of a percent more efficiency. At the same time an improvement in overall efficiency of the vehicle of 1% only implies about 3 miles extra range and as improvement in the regen system to 100% would not give even that 1% overall it isn't likely that any regen scheme is going to give a notable improvement over what is available now.
The Tesla system, as hypothesized, sends the energy directly back to the battery. Small capacitors (pF) would be used to absorb switching transients. But not the supercapacitors which are 15 orders of magnitude larger.